In general, an electrically heated textile is a structure having an electric heating element. The heating element may, for example, be heated by resistance via electricity, and may be provided as one or more metallic wires threaded throughout the pad. The shape and size of the metallic wires may vary, and in some cases the wires may actually be small metallic threads. The heating element typically includes a center heating element constructed of metallic wires having Positive Temperature Coefficient (PTC) characteristics. Around the center PTC wire is a layer of Negative Temperature Coefficient (NTC) material. An electric heating pad is typically plugged into a power outlet so that power may be supplied to the heating element, causing the production of heat. In this manner, the electrically heated textile may be used to warm a desired area of the body. Contemporary heating pads usually include a controller and/or microprocessor which control the amount of heat output from the heating pad.
Some electrically heated textiles also include circuitry designed to detect hot spots. A hot spot occurs when the temperature of any portion of an electrically heated textile exceeds limits designed to prevent a thermal injury to an unsuspecting user. A sensing wire is of low resistance and is typically wound around the NTC layer and it is used provide a path for leakage current and monitor increases in leakage current in the NTC layer caused by heating. However, a break in the sensing wire may result in the inability of the controller to detect hot spots.
One known way to help detect a sensing wire break is to check the voltage of the sensing wire as power is initially applied to the product. When power is first applied, the wire is cold, relatively speaking. As the heating pad gets warmer, the NTC resistance will decrease. This change in resistance can be used to determine if the sensing wire is broken at or in close proximity to the end of the wire, at the point where it is attached to the controller connector. While this may validate the integrity of the sensing wire when the unit is first powered up, it does not work as a continuous “broken sensing wire” detector once the heating pad reaches operating temperatures. Nor, is it capable of detecting a break in the sensing wire that is not close to the end of the NTC/PTC wire where it connects to the controller connector.
Another sensing wire method involves connecting together the two ends of the sensing wire. In this case, compensation is made for a single brake anywhere along the NTC/PTC wire, and continuous detection of hot spots is not affected. However, it is not possible to know that a break in the sensing wire has occurred. Additionally, if a second wire break occurs, loss of hot spot detection is very possible. Specifically hot spot detection is lost between the two wire breaks.
It is therefore desirable to provide a detection system that will continuously check the integrity of the sensing wire. If the sensing wire breaks at any time during use and for any reason, the system will shut down and prevent the use of the product.